In manufacturing martensitic stainless steel tubes, the quality control is generally carried out so as to suppress or eliminate harmful defects, together with an inspection for assuring the quality, using a non-destructive inspection apparatus, such as an ultrasonic flaw detecting apparatus or the like. However, scales on the surface of the steel tube generates noise, and therefore, the ratio of the signal intensity representing the defects to the noise intensity (hereinafter referred to as “S/N ratio”) is deteriorated (decreased), thereby increasing the re-inspection work.
In particular, in the case when an air quenching (air-cooling quenching) is applied to suppress hardening cracks in manufacturing martensitic stainless steel tubes, thick and loose scales (i.e., scales containing a number of bubbles and voids) are formed, so that a reduced magnitude of the S/N ratio is obtained, compared with ordinary carbon steel tubes. In addition, a recent increase in the flaw detection level is more and more strongly required to detect flaws each having shallow depth, since an oil well is designed or so on the basis of the fracture toughness. Therefore, in the field of producing steel tubes for an oil well, it is of new and central importance that the precision of detecting defects in the non-destructive inspection (NDI) is enhanced (i.e., the S/N ratio is improved).
Traditionally, it has been pointed out that the noise signal in the non-destructive inspection results from the scales on the surface of a steel tube. In fact, there are many steps of heating in the process of producing the steel tube, thereby making it impossible to significantly reduce the amount of scales in an actual operation. Although it is possible to suppress the generation of scales, using an atmosphere controlled furnace, such an installation requires an extremely large installation cost.
A number of researches and developments have been made on the scale from the viewpoint of the structure thereof as well as of preventing the generation of flaws resulting from the scale. A method of manufacturing a martensitic stainless seamless steel tube has been disclosed, for instance, in Japanese Patent Application Publication No. 2001-96304, wherein the generation of flaws on the outside surface can be significantly reduced by boring a billet under conditions that the thickness and void rate of a scale inside layer (inner scale) generated on the billet are maintained within predetermined ranges.
Moreover, in Japanese Patent Application Publication No. 5-269507, a method of manufacturing a seamless steel tube has been disclosed, wherein a semi-finished product of stainless steel, i.e., a billet containing Cr at 12 wt % or more is rolled after heating in a heating furnace, and further rolled after heating in a re-heating furnace, and the scale thickness on the rolled material is maintained 10-100 μm on the entrance side of each rolling stand, so that the seizure flaws and streak-shaped flaws can be suppressed.
In Japanese Patent Application Publication No. 6-15343, a descaling method has been disclosed, in which high-pressure water is sprayed onto the outer surface of a rolling blank material, and scales are removed with a wire brush in order to reduce the number of pit flaws which are generated from the intrusion of scales into the surface of the rolling blank material.
Moreover, in Japanese Patent Application Publication No. 10-60538, a method of manufacturing 13 Cr stainless seamless steel tubes has been disclosed, wherein the steel tube has an oxidation layer having a high corrosion resistance and a decreased surface roughness, in which case, outer scale layers are removed by high pressure water, after forming outer and inner scale layers having a total thickness of 100 μm or more. In addition, a method of manufacturing the 13 Cr stainless seamless steel tube has been disclosed in Japanese Patent Application Publication No. 10-128412, wherein the steel tube is coated by as-is surfaces formed in hot-rolling, in which case, the tube is rolled after removing an outer scale layer with a descaler and to maintain an inner scale at a thickness of 0.1-50 μm, so that an excellent surface properties and corrosion resistance can be obtained.
However, it is found that there are few technologies in which the thickness of scale and/or the bubble/void content ratio is specified in order to enhance the precision in the defect detection by greatly reducing the intensity of the noise detected in the non-destructive inspection, especially in the ultrasonic test (UST).